absolute zero
–273 Celsius: the temperature at which no substance contains any heat energy.
alpha radiation
A nonpenetrating but potentially damaging form of radiation. Alpha particles are identical to the helium nucleus, containing two protons and two neutrons.
anthropic principle
The notion that asking why the universe is as we see it is essentially pointless because we could not exist in any universe that is substantially different.
antimatter
The “nemesis” of matter: every matter particle has an antimatter counterpart. When a particle and its antiparticle collide, they annihilate one another.
beta radiation
Radiation composed of high energy electrons or their antiparticles (positrons) that is readily stopped by a thin sheet of metal. It is caused by nuclear processes that involve the weak nuclear force.
Big Bang
The moment, nearly 14 billion years ago, when the universe came into existence.
black hole
A patch of space–time where gravity is so strong that nothing (not even light) can escape. Black holes are often formed when a giant star collapses under the pull of its own gravity.
boson
A particle with a quantum “spin” number that is an integer. Bosons are the particles through which forces act.
closed timelike curve
A region of space–time that, if followed, brings one back to the same moment in time.
compactification
The process by which the “extra” dimensions suggested by many modern theories of physics remain undetected.
cosmic microwave background radiation
The radiation released around 300,000 years after the Big Bang. The CMB radiation contains many clues to the nature of the early universe.
dark energy
A mysterious form of energy that many physicists believe to be responsible for the accelerating expansion of the universe.
dark matter
Hypothetical matter that, according to most astronomers, makes up nearly a quarter of the mass/energy in the universe. Dark matter is scattered throughout the universe, but concentrated in haloes around galaxies and galaxy clusters.
double slit experiment
An experiment originally created to demonstrate the wave nature of light. It has also been used to show that quantum particles have wave-like characteristics.
electromagnetic force
A force that causes particles with an electric charge to repel or attract one another, depending on the signs of their charge.
electron
A subatomic particle, believed to carry the fundamental unit of electric charge.
electroweak interaction
A force, thought to exist in the hot conditions of the early universe, which gave rise to the electromagnetic and weak forces.
entanglement
A phenomenon that occurs when two quantum particles interact. They become linked, with information about both particles residing in each one.
entropy
A measure of the disorder of a physical system. The entropy of a closed system always increases.
ether
A fluid that was thought to fill the universe and carry electromagnetic waves such as light. Its existence was disproved in 1887.
A matter particle whose quantum spin is half of a whole number.
fractal
A geometry that looks the same whatever the scale on which it is viewed.
fundamental forces
The four fundamental forces are: strong; electromagnetic; weak; gravity. See the table on page 204.
gamma radiation
A highly penetrating radiation that results from the emission of high-energy (gamma ray) photons in nuclear reactions.
gas
A fluid composed of particles that are only very weakly attracted to one another.
general relativity
Albert Einstein’s description of the warping of space and time by the presence of mass and energy.
geodynamo
The turbulent ball of churning molten iron that occupies the inner core of the Earth and creates a self-sustaining magnetic field.
gluon
The particle that mediates the strong nuclear force.
grand unified theories (GUTs)
Theories that attempt to define how three of the forces of nature (strong, weak and electromagnetic) were once one in the conditions of the early universe.
gravity
The attractive force that acts between particles with mass or energy.
Heisenberg uncertainty principle
A rule that imposes a limit on the accuracy with which certain combinations of properties can be known for any quantum particle or system.
hidden extra dimensions
Certain theories suggest that there are more spatial dimensions than the ones (up-down; back-forth; across) we experience.
Higgs boson
A hypothetical particle that is thought to mediate the Higgs field, the source of certain types of mass.
inflation
A theory that suggests the universe went through a period of ultrafast expansion just after the Big Bang.
interference
A wave phenomenon where two waves interact, producing a wave whose properties depend on the size and relative phase of the original waves.
inverse square law
When a force between two spatially separated objects diminishes, by an amount proportional to the square of the distance between them, as they are moved further apart.
kinetic energy
Energy associated with motion.
liquid
A state of matter in which particles have some attractive forces acting between them, and can only be further separated by an input of energy.
magnetosphere
The magnetic field surrounding the Earth.
many worlds theory
The idea, originated by Hugh Everett III, that every quantum event creates a new and separate universe. Only quantum particles such as electrons are able to sense the existence of these other worlds.
mass
The attribute of an object that gives resistance to acceleration (inertial mass) or responds to and creates the gravitational force (gravitational mass). Physicists believe the two types of mass are equivalent but this is not proven.
Maxwell’s equations
The equations, laid out by James Clerk Maxwell, that define how electric and magnetic fields behave and interact.
momentum
The product of the mass and velocity of a particle.
multiverse
A universe composed of myriad smaller universes, usually without any connection that allows passage between them.
neutron
A particle composed of three quarks, but with no net electric charge.
nucleus
The central core of an atom where almost all of its mass is concentrated.
particle accelerator
A machine used to smash subatomic particles into one another. Analysis of the resulting debris can give clues to the fundamental constituents of matter and the nature of the universe.
perpetual motion machine
A hypothetical machine that would do useful work with no energy being supplied.
photon
A particle of light or other electromagnetic energy.
potential energy
The energy an object contains because of its position within a field (usually gravitational or electric).
proton
A positively charged particle composed of three quarks.
quantum
A fundamental unit, originally of energy but now applied to anything indivisible, such as the electron charge, in subatomic physics.
quantum chromodynamics (QCD)
A theory that describes the interactions of quarks and gluons in nuclear physics.
quantum electrodynamics
A theory that describes the interaction of the electromagnetic force with matter.
quantum gravity
A theory that will bring together quantum theory and relativity to create an overarching description of how things behave over very small and very large scales. String theory is one attempt to build a quantum gravity theory.
quark
A subatomic particle associated with atomic nuclei. Protons and neutrons are each composed of three quarks.
Schrödinger equation
The equation used to define how matter behaves at subatomic scales.
solar wind
A stream of charged particles that fly off from the surface of the sun. The solar wind is responsible for the aurora borealis (the northern and southern lights).
solid
The state of matter where particles are strongly bound together in a rigid structure.
space–time
The four dimensional fabric of the universe. Any one point in space–time is known as an “event”: a specific time and location.
special relativity
Einstein’s 1905 theory that laid out a way for the laws of physics to be the same for all observers, regardless of how they were moving through space and time. He later generalized the theory to include the effects of gravitational fields.
standard model
The theory of physics that describes the properties of all known particles, and their interactions.
string theory
A description of the subatomic world where particles and forces are described by the vibrations of strings and loops of energy.
strong force
The short-ranged force that binds quarks into protons and neutrons, and overcomes the repulsion of the positively charged protons to hold atomic nuclei together.
superconductor
A material that offers no resistance to the flow of electrical current.
superfluid
A fluid that exhibits no friction. The motion of a stirred superfluid continues indefinitely.
A theory that gives known particles a hypothetical “superpartner” that is much heavier but has a related quantum spin. Supersymmetry is an important part of the attempts to unify all the forces of nature.
thermodynamics
The study of how energy will create heat, work and entropy in a physical system.
time dilation
A prediction of special relativity, borne out in experiments, that says the flow of time is altered by relative motion or by the presence of a gravitational field.
vacuum energy
The energy inherent in otherwise empty space–time. It is due to the Heisenberg uncertainty principle, which does not allow space–time to have zero energy for any finite period of time.
weak force
The short-range force that affects all matter particles. It is responsible for, among other things, some forms of radiation.
wormhole
A hypothetical shortcut through space–time that could be used for time travel.